The invention is directed to a capacitor ignition system for internal combustion engines, especially for lawn mowers, chain saws, abrasive cutting-off machines, etc., having a magnetic generator and a primary firing pulse generator for charging a capacitor. In this type of system the capacitor is discharged via the primary winding of an ignition coil by an electronic switch. The primary pulse generator is triggered by the magnetic generator. A tachometer addresses a stored characteristic ignition field program to control an rpm-dependent delay in the firing pulse relative to the magnetic generator pulse. The characteristic ignition field program has different advance curves for the same rpm under different operating conditions. A switching-over device is provided for switching over from a starting ignition advance curve to an operating ignition advance curve and back again.
In the capacitor ignition system of the above type, as disclosed in German Patent Application 38 17 471.5, a switching-over device that monitors the operating state is provided in order to switch over to an operating advance curve for which the advance increases as the rpm decreases, as the advance increases with increasing rpm and when a specified rpm is reached. After the engine has stopped for a specified time, the curve is switched back to the starting advance curve by means of a time function element that is activated by the stoppage of the engine. The switch-over point of the advance curve is below the idling rpm.
Such a construction for a capacitor ignition system accomplishes first of all that when the engine is cranked, essentially the starting advance curve is run through, for which the ignition time is relatively late in comparison to the ignition time for an operating rpm of about 8,000 for a chain saw or an abrasive cutting-off machine. This prevents too early an ignition pulse from driving the engine backward and simultaneously pulling the starting rope out of the hands of the operator. Of course the advance can be programmed in any way, so that it can be adapted to the particular use and also for the intended equipment. Running in the reverse direction can thus always be avoided by selecting the optimum advance for the particular engine.
If the gas lever is let go suddenly at full throttle (reach-me-down), the danger exists that the rpm will drop below the idling rpm and the engine will stall. For this reason, an operating advance curve which has been addressed, which deviates from the starting advance curve above the idling rpm and which rises with reverse slope toward lower rpm, is provided in the characteristic ignition field program. As soon as the engine starts up, it reaches the idling speed of, for example, 3,000 rpm. At the same time, the change-over point, that is, the point at which the two advance curves deviate from one another and which lies, for example, at 3,000 rpm, is traversed and the digital capacitor ignition system automatically switches over to the operating advance curve. If, however, the engine speed now drops below the idling speed, the engine receives early ignition pulses. This causes the engine output and thus the rpm to be increased, so that the engine does not stall.
If the engine is turned off and has been switched off for more than a specified stoppage time, this stoppage time being considered primarily as a safety measure, the system is switched with delay via a time-function element back to the starting advance curve, by way of a sensor, for example, a tachometer or an engine temperature gauge, which detects this operating state.
Admittedly, this prior capacitor ignition system reliably prevents the engine from stalling as the rpm drops. However, due to its rigid switching-over behavior, it is no longer adaptable to the respective requirements and, above all, can also not be operated by the user alternately for other functions of the ignition advance.